Over the past ten years, substantial efforts have been dedicated to the design of controllers for switching power supplies required to provide electrical power having both a high power factor and low total harmonic distortion (THD). One objective in designing such controllers has been to distort as little as possible the AC line current that is provided to such switching power supplies.
For a time, it was believed that the International Electronic Commission would subject electrical equipment, such as computers and communication devices, to a proposed standard IEC-555, which called for THD limits as low as 5%. However, the IEC never adopted IEC-555, opting instead to approve IEC-1000.3.2 (incorporated herein by reference) imposing less-rigorous THD limits.
For power supplies operating at about 1.5 kW (at 110 VAC or about 3 KW at 220 VAC), IEC-1000.3.2 limits the allowable input current THD to 5%, as did the proposed IEC-555 standard. However, for power supplies operating below 600 W, the IEC-1000.3.2 standard allows THD of up to 30%.
Peak current mode control and average current mode control are two well-known, alternative approaches to regulating the output voltage of switching power supplies to achieve high power factor. Presently, the industry-preferred approach is average current mode control.
In conventional average current mode control, the control voltage (representing an error between the actual output voltage and a reference voltage) is used to produce a reference current to modulate a desired input current. Therefore, the control voltage directly controls the output voltage. Ideally, the control voltage in the conventional average current mode controller directly controls the average value of the current passing through the inductor for the fastest response.
Unfortunately, the conventional average current mode control is more complex and expensive to implement than is alternative peak current mode control. Additionally, high power factors and lower THD are attainable using peak current mode control for narrow input voltages and load variations. See, "An Overview of Power Factor Correction in Single-Phase Off-Line Power Supply Systems" by J. Sebastian, et al., IECON94 Proceedings, incorporated herein by reference.
In peak current mode control, the switch or inductor current is compared with a threshold signal. THD of less than 5% is realizable if the parameters of the ramp waveform are appropriately selected for the particular line current. Peak current mode control, however, has difficulties in providing low line current distortions over a wide range of AC input voltages and output loads.
Various enhancements extending the circuit design of basic peak current mode control have attempted to adjust the circuit parameters of the ramp waveform to extend the practical operating range of peak current mode control. These enhancements include feed-forward from the rectified AC line voltage to correct the parameters of the ramp waveform. Another approach uses the output of the voltage amplifier to correct the ramp, so as to increase the practical range of operation in discontinuous conduction mode (DCM). See, "Power Factor Correction Using a Preregulating Boost Converter" by L. Hadley, Power Conversion Proceedings, October 1989, pp. 376-382, and "Reducing Distortion in Boost Rectifiers with Automatic Control" by R. Redl, Applied Power Electronics Conference Proceedings, 1997, pp. 74-80, which are incorporated herein by reference.
However, the techniques and enhancements described in the above-mentioned publications are only applicable to converters, i.e., boost converters, operating in DCM. Similar problems and limitations faced by converters operating in continuous conduction mode (CCM) are not addressed. Furthermore, although those techniques discussed above have been able to improve the operating range of the power factor correction stage in DCM of operation, there are still regions of very high THD (greater than 40%) for power supplies required to operate from 85 VAC to 265 VAC (commonly known as "universal input").
Accordingly, what is needed in the art is an improvement to current mode control that permits CCM operation and provides lower THD over a wider range of input voltages and load variations.